The present invention relates to medical stimulators and leads generally, and more particularly to implantable defibrillators and defibrillation leads.
Currently available implantable ventricular defibrillators typically employ epicardial or subcutaneous patch electrodes, alone, or in conjunction with one or more transvenous electrodes. Multi-electrode ventricular defibrillation systems are disclosed in U.S. Pat. No. 4,727,877 issued to Kallok, U.S. Pat. No. 4,708,145 issued to Tacker, et al., and as disclosed in U.S. Pat. No. 5,099,838, issued to Bardy. Other endocardial defibrillation electrodes are disclosed in U.S. Pat. No. 4,481,953 issued to Gold et al., U.S. Pat. No. 4,161,952 issued to Kinney, et al., U.S. Pat. No. 4,934,049 issued to Kiekhafer et al. and in U.S. Pat. No. 5,042,143 issued to Holleman, et al. The Kinney, Gold, Holleman and Kiekhafer patents all disclose endocardial defibrillation leads employing defibrillation electrodes fabricated from elongated coils of biocompatible metal, mounted exposed to the exterior of the defibrillation lead, for location in the right ventricle and other locations within the heart. The above-cited Smits patent and the Mehra application both disclose a variety of endocardial defibrillation electrodes intended for use in the atrium, ventricle and coronary sinus, all of which employ electrodes taking the form of elongated coils of conductive biocompatible metals. U.S. Pat. No. 4,922,927, issued to Fine et al. proposes the use of a ventricular defibrillation electrode system using only a right ventricular electrode and a subcutaneous electrode, which may correspond to prior art subcutaneous electrodes or may be the metal enclosure of the defibrillator.
Concurrent with the development of lead systems adapted to treat ventricular fibrillation, there has also been some work directed to the development of lead systems to treat atrial fibrillation. Synchronized cardioversion using two electrodes located on a lead located in the right atrium is disclosed in U.S. Pat. No. 3,738,370, issued to Charms. A later system is disclosed in U.S. Pat. No. 3,952,750, issued to Mirowski et al., employing one electrode in the atrium and presumably a second electrode at an unspecified location. Neither of these references discloses a specific embodiment for the electrodes located in the atrium.
An electrode lead system specifically designed for atrial defibrillation is disclosed in the article "Elective Countershock in Atrial Fibrillation With an Intracardiac Electrode--A Preliminary Report, by Jain, et al., published in the Journal of the Association of Physicians of India, Vol. 18, pp 821-824, 1970. This lead was provided with a 10 mm silver electrode for location in the right atrium and was tested in conjunction with either a second electrode located in the right atrium or a second, cutaneous electrode located on the left side of the chest wall. A second electrode system specifically designed for use in atrial cardioversion is disclosed in the article "Safety and feasibility of transvenous cardioversion in atrial tachycardia", by Blanc et al., published in Cardiac Pacing, edited by Gomez, Futura Pub. Co., 1985, pp 1526-1529. This electrode system employed a single lead with electrodes located in the atrium and pulmonary artery. More recently, the use of electrodes located in the right atrium and coronary sinus/great vein for atrial defibrillation has been disclosed in U.S. Pat. No. 5,165,403 issued Nov. 24, 1992 to Mehra. Delivery of atrial defibrillation pulses between the right ventricle and a subcutaneous electrode is disclosed in U.S. Pat. No. 5,292,338, issued Mar. 8, 1994 to Bardy. Delivery of atrial defibrillation pulse between a coronary sinus electrode and a subcutaneous electrode is disclosed in U.S. Pat. No. 5,314,430, issued on May 24, 1994 to Bardy. The cited Mehra patent and the two cited Bardy patents are hereby incorporated herein by reference in their entireties.
In the context of an implantable atrial defibrillator, it is especially desirable to reduce defibrillation energy thresholds. Because it is anticipated that such devices will likely deliver defibrillation pulses more frequently than implantable ventricular defibrillators, reduced energy thresholds are necessary to achieve a device having acceptable longevity. In addition, as the patients receiving the defibrillation pulses will generally be conscious, frequent painful shocks are believed to be undesirable, if not unacceptable. This factor further reduces the desirable atrial defibrillation threshold to about 1 Joule or less. Despite the amount of activity in this area, as reflected in the references cited above, the goal of a defibrillation lead system which will generally accomplish such low defibrillation thresholds has not been accomplished. The right atrium to coronary sinus/great vein pathway, disclosed in the Mehra '403 patent accomplishes this goal in some patients, but in others requires substantially higher energy levels.